Identifying the proper degree of federal supremacy and the best means of building it into the constitutional structure were central concerns for many members of the founding generation. At the Constitutional Convention, James Madison proposed granting Congress the power to veto state legislation. Madison’s “negative” was intended to connect Congress and the states in a single compound legislature, giving Congress the power either to veto or to ratify by silence the acts of state legislatures. The negative failed to gain the approval of the convention delegates, however, and they instead chose to build federal supremacy into the Constitution via the judiciary-centered mechanisms of the Supremacy Clause and Article III. This essay asks what would have happened if Madison’s negative had carried the day, and the Constitution had implemented federal supremacy by way of a legislative rather than a judicial device. One potential answer is that the negative should be understood as the functional equivalent of modern preemption doctrine. Had the negative been incorporated into the Constitution in 1787, however, the combined force of the negative’s distinctive characteristics might well have led not to a stronger union but to forceful resistance to federal power by diverse state legislatures in a variety of circumstances. Two nineteenth-century case studies illustrate this point: the controversy over the Bank of the United States, and the debate over Congress’s power to supplant state legislation in the area of interstate commerce. In contrast to Madison’s and many modern commentators’ understanding of the negative as a highly centralizing mechanism, these case studies show that the negative would likely have led to fragmentation and disintegration between the federal center and the state peripheries long before the antebellum sectional crisis.